The invention relates generally to communications over the Public Switch Telephone Network (PSTN) including local subscriber loops and more particularly to a system and method of reducing interference in a splitterless DSL modem environment caused when telephone equipment coupled to a twisted pair wiring goes off-hook causing unwanted harmonics generated when the DSL signals encounter the off-hook circuitry in the telephone equipment.
The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the public switch telephone network (PSTN). An example includes the emerging modem communication standards which have pushed the limit of transmission speeds close to 56 Kbps. Another example includes the emerging variety of the DSL communications protocols including asymmetric digital subscriber line (ADSL), symmetric digital subscriber line (SDSL), high bit rate digital subscriber line (HDSL), and very high rate digital subscriber line (VDSL). Each DSL variant represents a different transmission speed over a different distance of copper pair wiring. Currently, industry is producing equipment capable of implementing one or more of the DSL protocols. Modems, routers, line cards, and digital loop carrier systems are all examples of such equipment.
In principle, a DSL modem and a plain old telephone system (POTS) or other voice band device can operate simultaneously over the same wire line pair since they use different frequency bands. The connection, however, of a POTS equipment to the same wire line pair as a DSL modem can suffer from several problems that result from the change in input impedance of the POTS equipment as it is added to the line. In essence, a POTS equipment is not designed to handle frequencies outside the voice band of 300-3.4 kHz. In addition, a POTS often contains nonlinear components that may create harmonic interferences. Examples of such nonlinear components include Zener diodes, transistors, varistors, triacs, and other devices used for overvoltage protection, sidetone generation and overvolume protection.
Splitterless modems try to take advantage of this principle to transmit both voice band and digital DSL signals simultaneously. An example of such a splitterless modem architecture is disclosed in related U.S. patent application Ser. No. 09/216,082 (the xe2x80x9cRelated Applicationxe2x80x9d), entitled xe2x80x9cResidential Power Cut-back for Splitterless DSL Operationxe2x80x9d.
In a splitterless configuration, when the POTS goes off-hook during a DSL transmission, these components can distort the DSL signal causing intermodulation and producing nonlinear echos of the upstream DSL signal. Harmonics of these echos lie in the downstream DSL band, and thus severely corrupt the downstream signal.
Therefore, when a splitter is not utilized to separate the ADSL modem and the Plain Old Telephone System (POTS), the DSL and the POTS equipment will generate interference overlapping their respective frequency bands. For example, assuming the DSL modem transmits upstream from approximately 30 kHz to 140 kHz, the nonlinear circuitry in the POTS equipment may affect the upstream signals once the POTS goes off-hook. The harmonics generated by the DSL signal encountering the POTS equipment contributes to the distortion and noise over the communications link resulting in loss of data and/or a reduction in the upstream data rate.
Since discrete multi-tone modulation is the preferred method of line coding in a standard DSL system, there are many subbands in both the upstream and downstream directions which can be effected by the harmonic interference caused when a POTS device goes off-hook. In a splitterless environment, the harmonics from the off-hook telephone set are likely to be distributed over all the tones in both directions. Thus, a method of detecting the off-hook state of the telephone set and placing the DSL transceiver in an alternative operating state could help reduce or deal with the effects of such harmonics.
The present invention provides a DSL communications device capable of detecting harmonics generated when a Plain Old Telephone System (POTS) device coupled to the same wire line pair as the DSL device goes off-hook. A threshold level of power in each tone can be established so that the DSL device is able to switch between states in which the modem parameters are optimized to work with the POTS equipment in either the on-hook or off-hook state. In the off-hook state, techniques are applied that reduce the unwanted harmonics in the upstream and downstream paths. By transmitting data on fewer tones of the upstream channel, an increase in the number of downstream channels is realized.
In one embodiment, only the upper sub-band tones in the upstream direction are utilized at lower power levels to reduce the effects of unwanted harmonics in the downstream tones and thereby reduce the effects of voiceband interference. While the upstream data rate may be reduced, the communications link is maintained and the downstream data rate continues as high as possible. In essence, the physical characteristics of the distortion generated by the nonlinear components in the POTS equipment is exploited to achieve high throughput.
Also disclosed is a method of reducing the effects of harmonics generated when a POTS device goes off-hook comprising the steps of monitoring the harmonics of DSL signals generated over said communications link and controlling the tones used to communicate said DSL signals over said communications link communications network with a, splitterless Digital Subscriber Line (DSL) modem and a Plain Old Telephone System (POTS) device communicating over the same communications link.